The applicant has already proposed, for example in patent FR 2 912 893, then in the patents FR 2 994 819, FR 2 994 820, or FR 2 994 821, an electrophysiological analysis system comprising a series of electrodes intended to be positioned in different regions of the body of a patient, a DC voltage source adapted to generate adjustable DC voltage pulses, and a control circuit arranged to selectively connect a pair of so-called active electrodes to the source of Voltage electrodes, said active electrodes constituting an anode and a cathode, and for connecting at least one of the other high impedance electrodes. The voltage applied by the voltage source to the electrodes makes it possible to generate an electrophysiological current in the outer layer of the skin by electrochemical reaction at the level of the sweat glands, the study of certain characteristics of which may indicate certain pathological pathologies or predispositions. The sweat glands, of which a schematic electrical representation is given in FIG. 1, behave as a non-linear dipole, that is to say that the resistance Re (and therefore the conductance) of the sweat glands varies as a function of the difference of V′e−Vx, where Vx is the potential of the body, which is given by measuring the potential of the electrodes connected in high impedance, V′e is the voltage imposed on the gland by an electrode, Ve is the Voltage applied or measured at the electrode, and Ee=Ve−V′e is the force against electromotive force or overvoltage of the electrode.
The conductance of the sweat glands, or electrochemical conductance of the skin, is therefore the ratio between the current flowing through said glands and the potential difference to which they are subjected. However, this conductance varies not only as a function of the difference in potential applied to the sweat glands, but also as a function of the state of health of the individual. Thus, for example, with reference to FIG. 2a, the curve of the current passing through the eccrine glands is represented as a function of the potential difference imposed on the glands present in a healthy patient. There are two distinct sections: a first linear section, followed by a detachment and a second non-linear section.
The detachment corresponds, in FIG. 2b, to a voltage threshold beyond which the conductance is increasing with the potential difference. On the other hand, this detachment may disappear in certain individuals suffering from pathologies, such as cystic fibrosis. This has been developed in the Applicant's patent application US-2013-0053673.
It is therefore interesting to measure the conductance for different potential differences imposed on the electrodes in different persons, to diagnose diseases in these persons, or several times in the same person, to monitor the course of a disease. However, the chemical reactions stimulated in the skin of an individual by the application of voltage to electrodes lead to oxidation and reduction reactions at the level of the electrodes themselves. The oxidation reactions cause the existence of an overvoltage at the level of the electrodes, having the effect of biasing the measurements of voltages at the level of the electrodes and therefore of degrading the quality of the electrochemical conductance measurements of the skin.
This problem has already been raised in patent application FR 1358780 by the Applicant. In this patent application a specific measurement protocol has been proposed to stabilize the overvoltage appearing on an electrode the time of a measurement in order to correct the data obtained. However, the overvoltage of an electrode can change over time and it is not sufficient to stabilize this overvoltage only for the time of a measurement. Indeed, it is not possible to compare data acquired in one or more persons if the overvoltage of the electrodes used to obtain the data has evolved.